Glitch-aware phase algebra for clock analysis

ABSTRACT

In some embodiments, a method for processing register transfer level code representing a circuit design. The method can include determining, by one or more processors based on the register transfer level code, an input sequence of signal transitions associated with an input net of a component represented in the register transfer level code, wherein each signal transition represents a nondeterministic transition from a first signal state to one or more possible signal states; determining, at least one of the processors based on the register transfer level code, that a subsequence of signal transitions of the input sequence indicates at most one transition within the subsequence; and determining, by at least one of the processors based on the component represented in the register transfer level code and on the subsequence, an output sequence of signal transitions derived from the input sequence of signal transition.

RELATED APPLICATIONS

This application claims priority from a U.S. patent application Ser. No.14/547,532, filed on Nov. 19, 2014, which claims priority from a U.S.Provisional Patent Application Ser. No. 61/912,345, filed on Dec. 5,2013, which is incorporated herein by reference. This application isalso related to a U.S. patent application Ser. No. 14/327,658, filed onJul. 10, 2014, entitled “CIRCUIT DESIGN EVALUATION WITH COMPACTMULTI-WAVEFORM REPRESENTATIONS,” which is incorporated herein byreference.

BACKGROUND

Embodiments of the inventive subject matter generally relate to thefield of circuit design, and, more particularly, to electronic designautomation (EDA) tools to identify potential defects in a registertransfer level (RTL) design of a chip or a system on a chip.

EDA tools are used to evaluate chip designs prior to fabrication. TheEDA process broadly consists of two steps. The first step is a check ofthe RTL design logic. The second step is a creation of a physicalcircuit design from the RTL design. The first step, checking the designlogic, can be referred to as RTL design checking. In RTL designchecking, a language such as VHDL (Very High Speed Integrated CircuitHardware Descriptive Language) or Verilog can be used to describe andmodel the functional behavior of a circuit. RTL design checking itselfcan be decomposed into two steps. The first step is static checking andthe second step is verification, also commonly referred to as a dynamicchecking. In static checking, the structure of the design is analyzedwithout simulating the behavior of the design. Conversely, inverification, the design is simulated by applying test patterns orstimulus to the inputs of the design in an attempt to exhaustivelyidentify possible errors. Verification can be an expensive process for acomplex chip or system on a chip. Verification can also be inconclusive,since it is often infeasible to apply all possible test patterns to theinputs of a complex design.

Chips and systems on chips continue to increase in complexity,comprising many systems and sub-systems. These systems and sub-systemsmight comprise multiple clock domains. A clock domain is a set ofsequential logic elements, such as transparent latches and flip-flops,and combinational logic associated with these sequential logic elementsthat are clocked by a common clock or by clocks having common frequencyand a fixed phase relationship. A clock signal causes a change in thestate of sequential logic, such as a flip-flop or transparent latch. Anasynchronous clock domain crossing is a path from a sequential logicelement or other source of state transitions in a design in a firstclock domain to a sequential element in a second clock domain throughwhich transitions may occur when the first clock domain operatesasynchronously with respect to the second clock domain. When a datasignal crosses from a first clock domain to a second clock domain andthe first clock domain is asynchronous to the second clock domain, thecrossing is referred to as an asynchronous clock domain crossing.

SUMMARY

In some embodiments, a method for processing register transfer levelcode representing a circuit design. The method can include determining,by one or more processors based on the register transfer level code, aninput sequence of signal transitions associated with an input net of acomponent represented in the register transfer level code, wherein eachsignal transition represents a nondeterministic transition from a firstsignal state to one or more possible signal states; determining, atleast one of the processors based on the register transfer level code,that a subsequence of signal transitions of the input sequence indicatesat most one transition within the subsequence; and determining, by atleast one of the processors based on the component represented in theregister transfer level code and on the subsequence, an output sequenceof signal transitions derived from the input sequence of signaltransition.

BRIEF DESCRIPTION OF THE DRAWINGS

The present embodiments may be better understood, and numerous objects,features, and advantages made apparent to those skilled in the art byreferencing the accompanying drawings.

FIG. 1 is a conceptual diagram that depicts example phase algebra basedcircuit design evaluation with a compact multi-waveform representation.

FIG. 2 illustrates the relationships among the G-function, a waveform,and the M-function.

FIG. 3 depicts an example conceptual mapping of transitions in multiplewaveforms to NTFs.

FIGS. 4A-4B depict example NTF sequences and the information encoded inthe sequences.

FIG. 5 is a conceptual diagram that depicts an example hierarchy ofrelationships among data constructs.

FIG. 6 is a flowchart of example operations for initializing an RTLcircuit design representation of phase algebra based evaluation andpropagation of compact multi-waveform representations throughout thedesign representation.

FIG. 7 illustrates terminology associated with the example propagationalgorithm and pseudocode to be described.

FIG. 8 is a flowchart of example operations for initializing a circuitdesign representation for phase algebra based evaluation.

FIG. 9 is a flowchart of example operations for determining an outputmulti-waveform representation based on an input sequence ofnondeterministic transition representations.

FIGS. 10 and 11 are flowcharts of example operations for applyingcircuit component based operations to sequences of signal transitionrepresentations.

FIG. 12 is a flowchart of example operations for implementing thecwss_fix_latch operator.

FIG. 13 is a flowchart of example operations for implementation of thecwss_path_delay operator.

FIGS. 14A-14B depict a flowchart of example operations forimplementation of the cwss_is_subset operator.

FIG. 15 is a depiction of circuit design evaluation with example phasetags.

FIG. 16 depicts an example computer system compact multi-wave basedcircuit design evaluator.

FIGS. 17A-J show tables described in the specification.

DESCRIPTION OF EMBODIMENT(S)

The description that follows includes example systems, methods,techniques, instruction sequences and computer program products thatembody techniques of the present disclosure. However, it is understoodthat the described embodiments may be practiced without these specificdetails. For instance, the syntax employed to implement the disclosurecan be varied. Additionally, although illustrations refer to a flip-flopas a fundamental circuit component, embodiments need not include aflip-flop. For example, a circuit model can include transparent latchesand an inverter instead of a flip-flop as fundamental circuitcomponents. Additionally, embodiments may implement fewer operationsthan the operations described herein, while other embodiments might beimplemented with more operations that the ones described herein. Inother instances, well-known instruction instances, protocols, structuresand techniques have not been shown in detail in order not to obfuscatethe description.

Modern processors or systems on a chip include multiple components.Identifying as many design defects as possible at the static checkingphase of an RTL design check increases the efficiency of theverification process, thereby saving time and money. A design tool canimplement phase algebra based design evaluation as described herein toefficiently evaluate a circuit design with a compact representation ofnumerous waveforms without simulating the individual waveforms. Insteadof individual waveforms, the phase algebra based design evaluationemploys compact representations of a group or set of waveforms.

Phase algebra based evaluation constructs representations of a set ofwaveforms based on relationships among a devised set of functions thataccount for the various states of a signal over time, includingtransitions and glitches. A memorized-transition function, referred toherein as an M-function, indicates signal transitions over time. Theoutput value of the M-function indicates that a transition isoccurring/has occurred (e.g., indicated with a value of 1), or notransition has occurred (e.g., indicated with a value of 0) with respectto a given time interval. The M-function can also indicate (e.g., withthe value of 0) that the given time interval is outside a range ofinterest.

A glitch aware function, referred to herein as a G-function, accountsfor the occurrence of a glitch. A glitch can be characterized as atemporary deviation of a signal from its functional value. In general, aglitch occurs due to delays in inputs to a circuit component, delayinherent in a circuit component changing its output to reflect changesto its input, or both. For example, consider a first input and a secondinput to an AND gate. Assume that the first input at time t=1 isexpected to transition to a 1 and the second input at time t=1 isexpected to transition to a 0. However, if the second input is delayed,such that at time t=1, the second input is a 1 rather than a 0, then theoutput of the AND gate will be a 1 rather than a 0 as anticipated. TheG-function assumes a value of 1 for all times during which there isinterest in determining whether a glitch can occur. The relationshipsamong these functions are depicted in FIG. 2 later.

The glitch aware algebra distinguishes between signals that areglitch-free and signals that are glitch-prone. Thus, the evaluation toolcan identify some compact multi-waveform representations as beingglitch-free or as being glitch-prone. If the glitch aware algebra isbeing employed, the evaluation tool will propagate compactmulti-waveform representations on nets throughout a designrepresentation using look up tables that are constructed based on a setof possible waveform states, and both the M-function and the G-function.The glitch aware algebra can use additional elements of these look uptables, as described herein. For example, the glitch aware algebra canutilize a path delay block (PDB) of Table 1, as well as additionalelements of various other look-up tables described herein.

Furthermore, clock-gating algebra identifies signals that combine dataand clock signals to generate clocks that can be turned off, or that canbe forced on at certain times. The clock gating algebra alsodistinguishes among gated clocks that are gated low, high, or both.Thus, the evaluation tool can identify some compact multi-waveformrepresentations as being of an ungated clock type or as gated clocktype, and also distinguish between different types of a gated clock typeof a compact multi-waveform representation. Clock gating typicallyrefers to a type of digital logic that combines data and clock signalsto generate clocks which can be turned off or forced on at certaintimes. A low-gated clock is a clock that is low when gated. A high-gatedclock is a clock that is high when gated. A double-gated clock is aclock which can be either low or high when gated.

FIG. 1 is a conceptual diagram that depicts example phase algebra basedcircuit design evaluation with a compact multi-waveform representation.A design tool performs phase algebra based circuit design evaluation ona machine readable representation of an RTL circuit design 128. The RTLcircuit design representation 128 at least includes a primary input 112and a clock generator 116. A primary input 112 is an input to thecircuit itself. A primary input 112 is not driven by any componentwithin the circuit. The clock generator 116 represents a clock signalthat is provided to the circuit.

Table 1 (shown in FIG. 17A) depicts example RTL circuit designcomponents that can be modeled in an RTL circuit design evaluated by aphase algebra based evaluation tool (also referred to herein as anevaluation tool). In some embodiments, both the glitch aware algebra andthe clock gating algebra can utilize a path delay block (PDB) of Table1.

A compact multi-waveform representation 104 is provided for the RTLcircuit design 108. For example, the multi-waveform representation 104is provided in a RTL description using attributes or in a file suppliedas input to the evaluation tool. The evaluation tool determines compactmulti-waveform representations generated on nets throughout the RTLcircuit design dependent upon the components traversed by the compactmulti-waveform representations. Example notations “A” 120 and “A@L” 124for compact multi-waveform representations are depicted. These notationsare referred to as “phase tags” herein. This example uses this phase tagto illustrate handling of a virtual clock identified as ‘A’.

In this description, a phase tag and a phase type are distinguished. Aphase type is a construct (e.g., variable or notation) that represents ageneric virtual clock. Use of a phase type would be sufficient in adesign that contemplates a single virtual clock. A phase tag is aconstruct that identifies a virtual clock. Although a phase tag can beused in a design that contemplates a single virtual clock, the utilityof the phase tag becomes apparent when multiple virtual clocks are beingconsidered. In addition, operators associated with phase tags (“phasetag operators”) can manipulate results of phase type operators asappropriate for multiple virtual clocks. The particular terminology usedto distinguish these constructs should not be used to limit claim scope.For this illustration, the notation “A” represents a set of signals orwaveforms with a clock signal behavior corresponding to a virtual clockA. The notation “A@L” represents a set of signals or waveformscorresponding to a latch clocked by the leading edge of the virtualclock identified as A. The tables for phase types use the notation ‘C’as a general identifier of any virtual clock. The majority of thisdescription focuses on use of phase types and phase type operators.

The circuit design representation 128 also includes an inverter 132 andan AND gate 140. A net 148 is graphically depicted as connecting outputof the clock generator 116 to input into the inverter 132. A net 131 isgraphically depicted connecting output of the inverter 132 to a firstinput of the AND gate 140. A net 152 is graphically depicted asconnecting output of the primary input element 112 to a second input ofthe AND gate 140. The evaluation tool determines that inputting thecompact multi-waveform representation noted by the notation 120 into theinverter 132 will yield a compact multi-waveform representation with anotation “˜A” 126, which represents a set of signals or waveforms withan inverted clock signal behavior. The evaluation tool determines thatinputting the compact multi-waveform representation noted by thenotation 124 and the compact multi-waveform representation noted withthe notation 126 into the AND gate 140 will yield a compactmulti-waveform representation with a notation “*” with the basic phasealgebra since the basic phase algebra does not have values defined tohandle a gated clock. The notation “*” represents all sets of waveforms.

If the clock gating algebra is being employed, then the notation yieldedwould be “(˜A) %*” 126. The notation (˜A) %* represents a set ofwaveforms characterized as a low-gated inverted clock. The evaluationtool propagates compact multi-waveform representations throughout netsof the circuit design representation 128 using look up tablesconstructed based, at least in part, on a set of possible waveformstates. The clock gating algebra can use the elements of the glitchaware algebra (i.e., such as the PDB and other elements of the look uptables), as well as additional elements of various other lookup tables.

When compact multi-waveform representations have been determined, achecking unit 172 of the evaluation tool analyzes the compactmulti-waveform representations associated with the nets of the designrepresentation 128. The checking unit 172 can identify defects in thedesign using these compact multi-waveform representations.

For example, the checking unit 172 will evaluate the transition behaviorrepresented by a compact multi-waveform representation associated with anet against a rule or constraint of the net. The rule or constraint ofthe net can be explicit (e.g., directly defined in associated with thenet) or implicit (e.g., indirectly associated with the net via acharacteristic of the net or at least one of the sinks of the net).

Referring back to Table 1, the first column of Table 1 includes commonnames for the example components and the second column of Table 1includes symbols that commonly represent the example circuit components.The third column of Table 1 indicates the relationships between inputsto the circuit components and the outputs that the circuit componentsgenerate based on the inputs furnished to the circuit components. Thetransparent latch of row six of Table 1 is an example circuit component.Row six, column three of Table 1 specifies the relationship between theinputs to the transparent latch and the output that the transparentlatch generates. The transparent latch takes two inputs: a data signal,represented by D, and a clock signal, represented by C. The clock signalcan be generated by a clock generator, listed in row 12 of Table 1 orother harmonic oscillator. The transparent latch samples the data signalD when the clock signal equals 1. Thus, the output Q of the transparentlatch at time t, denoted Q(t), equals the data value D at time t−1,denoted D(t−1), when the clock at time t−1 takes a value of 1, denotedC(t−1)=1. Otherwise, the output Q of the transparent latch does notchange. In another embodiment, the transparent latch samples the datasignal D at all times during which the clock signal assumes a value of 0rather than a value of 1.

The flip-flop, shown in row seven of Table 1, is another circuitcomponent. Like the transparent latch, the flip-flop receives twoinputs, a data signal D and a clock signal C. The output Q of theflip-flop equals the value of the data signal. The flip-flop samples thedata signal only during a small interval of time when the clock signaltransitions from a 0 to a 1, unlike the transparent latch, whichcontinuously samples the data signal when the clock signal equals a 1.Thus, if the data signal at the time at which the clock transitions froma 0 to a 1 is a 0, then the output of the flip-flop will become a 0.Otherwise, if the data signal at the time at which the clock transitionsfrom a 0 to a 1 is a 1, then the output of the flip-flop will becomea 1. Column three of Table 1 specifies this relationship. The output ofthe flip-flop Q(t) at time t equals the value of the data signal at timet−1, denoted D(t−1), if the clock signal at time t−1 denoted C(t−1)=1,and the clock signal at time t−2, denoted C(t−2)=0, thereby signifying atransition in the clock signal from a 0 to a 1. The flip-flop can bemodeled by two transparent latches. The transparent latch and flip-flopeach are modeled to include a unit delay such that the transparent latchand flip-flop express the output shown in column three of Table 1 aftera unit has elapsed from the time of receipt of an input that causes achange in state of the output.

The combinational logic components shown in Table 1, such as the ANDgates shown in row three, are modeled to have no delay between the timethat the component receives an input and the time that the componentgenerates an output based on the received input. However, acombinational logic circuit component will likely show a delay betweenreceiving an input and generating an output. To model such a delay, apath delay block (PDB) can be implemented. A PDB (row nine in Table 1)represents a nondeterministic, bounded delay on the time necessary for acombinational circuit component to generate an output based on receivedinputs. The foregoing timing assumptions associated with the circuitcomponents avoid modeling physical time, and allow modeling abstracttime. This conserves computational resources.

FIG. 2 illustrates the relationships among the G-function, a waveform,and the M-function. The relationship between the G-function (g(t)), theM-function (m(t)), and a waveform (w(t)) is given by the followingexpressions: if g(t)=0, then m(t)=0; otherwise, if g(t)=1, then m(t)=1if and only if w(t) does not equal w(t−1) or m(t−1)=1, where g(t) is thevalue of the G-function at time t, m(t) is the value of the M-functionat time t, and w(t) is the value of a waveform at time t. As depicted,the conditions for an M-function to equal 1 are that w(t) does not equalw(t−1), or that m(t−1) equals one. These conditions correspond totransitions in the modeled waveform w(t). With the M-function and theG-function, the previous state of a signal can be related to multiplepossible next states of the signal with a compact representation. Eachof these relationships is referred to herein as a nondeterministictransition function (“NTF”). FIG. 3 provides a diagram to helpillustrate how a sequence of NTFs can represent multiple waveforms in acompact manner. The term “nondeterministic” implies that zero or onetransitions can occur in a given NTF, as defined by the Function Tableof Table 2 (see FIG. 17B).

FIG. 3 depicts an example conceptual mapping of transitions in multiplewaveforms to NTFs. The mapping of time to NTFs is referred to as awaveform set function (WSF). A WSF 304 specifies a set of waveforms 312,316, 320, and 324. Each of the waveforms 312, 316, 320, and 324 assumesa value of either a 0 or a 1 at each instant of time. For example,waveform 312 assumes a value of 0 at all times before time t=0, and attimes t=0 through t=6, but transitions to a 1 at time t=7 and assumes avalue of 1 between times t=7 and t=9, and at all times after time t=9.These waveforms can be grouped together to form a single waveform set(compact multi-waveform), depicted by a waveform set diagram 308. Thewaveform set diagram 308 encodes information about the aggregatebehavior of the waveforms 312, 316, 320, and 324. Associated with thewaveform set diagram 308 is a waveform set function (WSF) 304. The WSF304 maps each unit of time to an NTF. Each NTF relates a previous stateof a waveform or set of waveforms to a set of possible next states.Although separated by a few layers of constructs, the compactmulti-waveform representations mentioned earlier are based uponsequences of these NTFs.

The NTFs, which can be considered the building blocks, will be firstdescribed. Constructs that build upon these NTFs will then be described.FIG. 5 depicts an example hierarchy of data constructs/objects builtupon NTFs revealing relationships between the previously mentioned phasetags and NTFs. FIG. 5 will be explored in more detail after describingthe data constructs individually. It is noted that the FG NTF is used bythe glitch-aware algebra to indicate glitch-free time ranges, i.e.,where only a single transition can occur within a subsequence of FGNTFs. As defined by the glitch-aware algebra, non-FG NTFs can beglitch-prone. The glitch-prone NTFs (non-FG NTFs) indicate that multipletransitions can occur within a range of non-FG NTFs.

The FG NTF is typically used in a subsequence of FG NTFs of a sequenceof NTFs. For example, in a sequence of NTFs of {FS, FG, FG, FG, FS},there can only be one transition in the subsequence of {FG, FG, FG}. Inanother sequence of NTFs of {FS, FX, FX, FX, FS}, there can be up tothree transitions in the subsequence of {FX, FX, FX}. Table 2 identifiesNTFs employed for phase algebra based RTL design evaluation. As shown inthe Function Table, the input (mp, wp) includes a don't care term (“x”)for the “mp” element for all non-FG NTFs, meaning that the calculationof the output of non-FG NTFs is not concerned with whether a transitionoccurred in a range of previous NTFs. The FG NTF defines its output as afunction of both “wp” (waveform) and “mp” (indication of whether atransition occurred in the current subsequence of FG NTFs). If the “mp”element indicates that a transition has occurred in one of the previousFG NTFs in the subsequence, then another transition is not permitted bythe current FG NTF.

The first column of Table 2 is a label given to each NTF. The label inthe first column of Table 2 is arbitrary. The second column of Table 2specifies the relationship between inputs to the NTF and outputs thatthe NTF generates based on the inputs provided to the NTF. The inputs toeach NTF include a previous waveform state denoted wp and a previousM-function state, denoted mp. The output of each NTF is a set ofpossible next waveform states, denoted WN. Each NTF specifies one valueof a G-function that equals 0 or 1, as noted in the third column ofTable 2. A G-value of 1 specifies that the NTF is “glitch-aware.”Columns four through eight of Table 2 include one or more adjacencyrestrictions. The adjacency restrictions specify that if an NTF appearsat time t−1, then certain NTFs are prohibited at time t, based on theNTF that appeared at time t−1. For example, NTF FF is prohibited at timet if the NTF at time t−1 had been either F0 or FL.

Additionally, each NTF is associated with a waveform set diagram symbol,shown in columns four through eight of Table 2, that can appear at timet given an NTF at time t−1. For example, the NTF F0, shown in row 1 ofTable 2, exhibits the waveform set diagram symbol depicted in column 6at time t if the NTF preceding F0 at time t−1 was FG. However, if theNTF preceding F0 at time t−1 was FF, FR, FS, or FX, then the waveformset diagram symbol of F0 is as depicted in column 8 of Table 2. Awaveform set diagram symbol illustrates the set of possible waveformstates at time t, including whether such states can differ from thestates at time t−1, indicated by a vertical line, which represents atransition or possible transition at time t. These NTFs are combined insequences to form compact multi-waveform representations that complywith the above defined adjacency restrictions. When propagating compactmulti-waveform representations throughout a design, the compactmulti-waveform representations are decomposed into the NTFs in order toapply the appropriate NTF operators upon the constituent NTFs. It isnoted that the FG NTF is used by the glitch-aware algebra to indicateglitch-free time ranges. It is also noted that the FF and FR NTFs areused by the clock-gating algebra to indicate a double-gated clock type.

Table 3 (shown in FIG. 17C-D) identifies the NTF operations.

The NTF operators correspond to operations of circuit components (e.g.,ntf_and) and some operations employed for coherency (e.g.,ntf_fix_adjacent and ntf_is_subset). The operations can be implementedwith look ups because the look up tables are constructed based on thesignal behavior represented by the NTFs and the foundational functionsthat capture transitional behavior. Table 3 identifies eight NTFoperators. The ntf_not operator determines the NTF produced at theoutput of an inverter based on the NTF provided to the input of aninverter. The ntf_and operation determines the NTF produced at theoutput of an AND gate, given NTFs provided at the inputs of an AND gate.The ntf_xor operator determines the NTF produced at the output of a XORgate given NTFs provided at the inputs of a XOR gate.

The ntf_trans_latch operator determines the NTF at the output of azero-delay transparent latch based on the NTFs provided at the inputs ofa zero-delay transparent latch. The ntf_trans_latch_const operatordetermines the NTF at the output of a zero delay transparent latch givena first NTF that corresponds to a constant data input to the zero-delaytransparent latch and a second NTF input that corresponds to the clockinput to the zero-delay transparent latch. The ntf_unit_delay_rangeoperator determines the NTF output of a unit delay component based onNTF inputs to the unit delay component. The ntf_is_subset operatordetermines whether a first NTF is a subset of a second NTF, returning avalue of true if a first NTF is a subset of a second NTF. Thentf_fix_adjacent operator returns an equivalent but valid value for asecond NTF when the second NTF follows a first NTF in time. Thus, thentf_fix_adjacent operator ensures that the adjacency restrictionsassociated with NTFs are observed.

Column four of Table 3 includes descriptions similar to the foregoing.Column two of Table 3 indicates example syntax that can be employed tocall the corresponding operator named in column one of Table 3. Table 3employs the syntax ntf output=ntf operator (ntf input1, ntf input2) fora dual input operator and ntf output=ntf operator (ntf input) for asingle input. Column three of Table 3 indicates the look up tablesassociated with each of the NTF operators of column one of Table 3. Thelook up tables of column three of Table 3 indicate the NTF outputgenerated based on the NTF input provided to each NTF operator listed incolumn one of Table 3. It is noted that Table 3 includes entries for theFG NTF, as used by the glitch-aware algebra. It is noted that Table 3includes entries for the FF and FR NTFs, which are used by theclock-gating algebra.

FIGS. 4A-4B depict example NTF sequences and the information encoded inthe sequences. In this description, a sequence of NTFs is referred to asa clocked waveform set specification (CWSS). FIG. 4A depicts a sequenceof NTFs relative to a virtual clock 404A. A virtual clock is a clockwaveform generated by a source that might be external to the circuitdesign being evaluated. Three timing parameters define any virtualclock: tP, denoting the period of a clock, which is the time intervalfrom any rising edge to the next; tD, denoting the duty cycle of theclock, which is the time interval from any rising edge to the nextfalling edge; and tO, denoting the offset of the clock, which is theearliest non-negative time at which the clock rises. Each CWSS consistsof eight NTFs. Each NTF is associated with a numerical value, rangingfrom 0 to 7, referred to as a CWSS slot. The CWSS slot identifies oneNTF within a CWSS. Each slot is associated with certain times within aWSF. Table 4 (shown in FIG. 17E) indicates how the timing parameters ofa virtual clock correlate to the constituent NTFs of a CWSS.

In particular, Table 4 relates each CWSS slot, shown in the firstcolumn, to a set of times related to virtual clock timing parametersindicated in the third column, which, in turn, relate to NTFs and WSFsindicated in the fourth column. FIG. 4B depicts a diagram of a set ofwaveforms 416C, which can be expressed as a CWSS 404C, consisting ofCWSS slots 408C, each CWSS slot 408C corresponding to an NTF 412C. InFIG. 4B, the notation tR refers to the time of a rising edge of avirtual clock associated with a CWSS, which also corresponds to slot 1of the CWSS in this depiction. The notation tF refers to the time of afalling edge of the same virtual clock, which also corresponds to slotfive of this CWSS.

The CWSS construct has operators that are abstractions of the NTFoperators. Table 5 (shown in FIG. 17F) identifies the CWSS operators.The CWSS operators correspond to operations of circuit components (e.g.,cwss_and) and some operations employed for coherency (e.g.,cwss_is_subset). As can be seen by the implementation definition column,these operators rely on algorithms (referred to as “separate algorithm”and “generic algorithm”). These algorithms invoke the NTF operators foreach NTF that constitutes a CWSS. These algorithms are discussed later.There are nine CWSS operators. These include cwss_not, cwss_and,cwss_xor, cwss_trans_latch, cwss_trans_latch_const, cwss_is_subset, andcwss_unit_delay, which correspond to the counterpart NTF operators butperform operations on CWSSs. The CWSS operator cwss_fix_latchcorresponds to the NTF operator ntf_fix_adjacent, but for a particularuse related to a transparent latch. The CWSS operator cwss_path_delaydetermines the CWSS generated by a path delay block.

Column one of Table 5 lists the CWSS operators. Column two of Table 5indicates example CWSS operator syntax. Column four of Table 5 describesthe function of each CWSS operator. Column three of Table 5 refers tothe algorithms that implement seven of the nine CWSS operators. In oneembodiment, the cwss_not operator can be implemented in a for loop thatiterates through each slot of the CWSS, invoking the ntf_not operatorfor each CWSS slot. Since the slots of a CWSS correspond to NTFs, theCWSS operators generally are implemented by calling NTF operators,manipulating each of the eight NTFs that comprise the CWSS. Thecwss_unit_delay operator is implemented through the equation listed inrow seven, column three of Table 5. It is noted that the cwss_fix_latch,the cwss_path_delay, and the cwss_is_subset CWSS operations are used byboth the glitch-aware algebra and by the clock-gating algebra.

Table 6 (shown in FIG. 17G-J) identifies 55 phase types andrelationships with CWSSs and phase type groups. Each phase type can beconsidered a construct (e.g., variable or notation) that represents aset of waveforms as a function of a non-specific virtual clock, asmentioned above. Column one of Table 6 is a number assigned to eachphase type. The assigned numbers can be referred to as phase type ordernumbers. Selection of a phase type as a result of an operation thatmodels a circuit component (e.g., an AND gate or inverter) conforms tothis defined order of phase types. Algorithms discussed later will referback to this ordering. Column two of Table 6 includes example symbolsassigned to each phase type. Column three of Table 6 indicates namesassigned to each phase type for ease of reference. Column four of Table6 includes a waveform set diagram illustrating the set of waveformsdesignated by the phase type. It is noted that phase tags 45-47 and49-51 are used by the glitch aware algebra. It is also noted that phasetags 15-44 are used by the clock-gating algebra.

The waveform set diagram is a function of a virtual clock, as indicatedby the markers tR and tF in column four of Table 6, which designate therising and falling times, respectively, of a virtual clock, and whichmay thus vary from one virtual clock to another. Accordingly, each phasetype symbol corresponds to a waveform set diagram. Thus, operationsperformed on a phase type are operations performed on a set ofwaveforms, each depicted in column four of Table 6. Column five of Table6 indicates a CWSS associated with each phase type. Each CWSS iscomprised of eight NTFs, which collectively represent the waveform setdiagram of column four of Table 6. Column six is a group name assignedto each phase type, referred to herein as phase type groups.

Table 7 (shown in FIG. 17K) identifies phase type groups (PTGS). A phasetype group represents one or more phase types. For example, the phasetype group GCE represents a grouping of two phase types. Phase typegroups can be used to differentiate among phase types that have the sameCWSS. Phase types are assigned to a phase type group based on theintended use of the phase type. For instance, certain phase types areimplemented to designate clock signals, while other phase types areimplemented to designate data signals or constant value signals. Columnone of Table 7 indicates example identifiers to designate each phasetype group. Column two of Table 7 indicates the phase types that aremembers of each phase type group. Column three of Table 7 provides adescription of each phase type group.

The combination of a phase type and a phase type group allows compactrepresentation of multiple waveforms based on NTFs and the adjacencyrestrictions placed on the NTFs. In other words, phase types arerepresentations of CWSSs, and phase type groups allow for CWSSs to beoverloaded. Tables 8 (shown in FIG. 17L) and Table 9 (shown in FIG. 17M)identify operators for phase types and phase type groups. Theseoperators invoke the previously defined CWSS and NTF operators. Thephase type operators included in Table 8 correspond to circuitcomponents and to NTF and CWSS operators. The phase type operatorsoperate upon the higher level construct of phase types by invoking theoperators of lower level constructs. Since phase types correspond tosets of waveforms, the phase type operators represent operations on setsof waveforms. In Table 8, seven example phase type operators are listedin column one: pt_not, pt_and, pt_xor, pt_latch, pt_flipflop,pt_unit_delay, and pt_path_delay.

The pt_not operator determines the phase type output of an invertergiven a phase type input. The pt_and operator determines the phase typeoutput of an AND gate given at least two phase type inputs. The pt_xoroperator determines the phase type output of an XOR gate given at leasttwo phase type inputs. The pt_latch operator determines the phase typeoutput of a transparent latch given a clock signal phase type input anda data signal phase type input. The pt_flipflop operator determines thephase type output of a flip-flop given a clock signal phase type inputand a data signal phase type input. The pt_unit_delay operatordetermines the phase type output of a unit delay given a phase typeinput. The pt_path_delay determines the phase type output of a pathdelay block given a phase type input. It is noted that the pt_latchoperator and the pt_path_delay operator are used by the glitch-aware andthe clock-gating algebra.

The foregoing description is included in column four of Table 8, whichalso describes the purpose of each phase type operator. Column two oftable 8 provides example syntax for the phase type operators. Theexample phase type operator syntax is as follows: pt_y=pt_operator(pt_input) for a single input phase type operator and pt_y=pt_operator(pt_input1, pt_input2) for a dual input phase type operator. Columnthree of Table 8 includes example pseudocode for implementing the phasetype operators. Each of the phase type operators calls a functionidentified as first_matching_pt function, which relies upon the orderingof phase types in Table 6.

The phase type operators listed in Table 8 each are comprised of a callto a corresponding CWSS operator and a phase type group operator. Forinstance, the pt_xor operator calls the first_matching_pt function. Thearguments to the first matching_pt function include the CWSS operatorcwss_xor and the phase type group operator ptg_xor. The arguments to thecwss_xor operator include the pt_to_cwss function, called for each phasetype input. The pt_to_cwss function converts a phase type to a CWSS. Thearguments to the ptg_xor operator include the pt to_ptg function, calledfor each phase type input. The pt_to_ptg function converts a phase typeto a phase type group. These conversions are based on the relationshipsidentified in Table 6, which can be encoded in accordance with variousdata structures. Each phase type is associated with a CWSS and a phasetype group. Each CWSS is comprised of NTFs. Each NTF is based upon aWSF, which, in turn, represents a multiple waveforms.

In some embodiments, the operation of the phase type operators may bemodified for clock-gating algebra. Below are four modifications to theoperation of the phase-type operators of Table 8.

If the clock-gating algebra is implemented, a latch phase of a phasetype input can be ignored. Specifically, if the AND or the XOR phasetype operator is selected (e.g., the 2-input pt_and or the pt_xor phasetype operator of Table 8), the following can be performed. If this phasetype operator has one phase type input that is a clock type phase type,and also has a second phase type input that is a latch type phase type,then the latch type phase type can be replaced with an unknown constantphase type (i.e., with the phase type designated by the “?” symbol). Asdescribed above with reference to Table 7, a latch type phase type caninclude a phase type belonging to the GLE, GLL, or the GLT phase typegroup. Based on Table 7, a clock type phase type can include a phasetype belonging to the GCE, GCI, GCL, GCT, GGE, GGI, GGL, or the GGTphase type group. The appropriate (i.e., the AND or the XOR) phaseoperator is then computed. As a result, an AND phase operator of pt_andas applied to phase-type inputs of (“C”, “C@L”) would produce the resultof “C %*” instead of “*”.

If the clock-gating algebra is implemented, a clock type input can bespeculatively propagated. Specifically, if the AND or the XOR phaseoperator is selected (e.g., the 2-input pt_and or the pt_xor phase typeoperator of Table 8), the following can be performed. If this phaseoperator has one phase type input that is a clock type phase type, andalso has a second phase type input of an unknown phase type (e.g., “-”),then the unknown phase type can be replaced with an unknown constantphase type (i.e., with the phase type designated by the “?” symbol). Asdescribed above, a clock type phase type can include a phase typebelonging to the GCE, GCI, GCL. GCT, GGE, GGI, GGL, or the GGT phasetype group. The appropriate (i.e., the AND or the XOR) phase operator isthen computed. This embodiment can be used in circuit designs thatinclude an output of a latch that feeds back to that latch's enable orclock input. This phase operator can generate a gated clock type result,thus allowing the gated clock type of a compact multi-waveformrepresentation to propagate through the gate, instead of using anunknown result.

If the clock-gating algebra is implemented, a common gating conditioncan be assumed. Specifically, if a binary phase type operator with twophase type inputs is used, and both of the phase type inputs are a gatedclock type phase type, then the following can be performed. The variablept_op can represent the binary phase type operator. The variables pt_aand pt_b can represent the two phase type inputs. The variable pt_y canrepresent the result of the phase type operator, according to thefollowing. Based on Table 6, a gated clock type phase type can include aphase type having an order number from 15 to 44, as shown in column oneof Table 6. First, each of the gated clock type phase types can beconverted to a corresponding ungated clock type phase type, such as toungated clock variables pt_a_ug and pt_b_ug. An ungated clock type phasetype can include a phase type having an order number from 5 to 14, asshown in column one of Table 6. An ungated clock type phase type can beobtained from a gated clock type phase type by subtracting 10, 20, or 30from the order number of the gated clock type phase type. For example, aphase-type input of a gated clock type phase type of “C!1%*”, which hasorder number 19 as shown in column one of Table 6, can be converted to aphase type input of an ungated clock type phase type of “C!1”, which hasorder number 9 as shown in column one of Table 6. Next, a temporaryphase type variable (e.g., pt_y_temp) is determined to be equal to theresult of the binary phase type operator (e.g., pt_op) being applied tothe ungated clock type variables. For example, the temporary phase typevariable pt_y_temp is set to the result of the binary phase typeoperator pt_op being applied to pt_a_ug and pt_b_ug. If the temporaryphase type variable is of a clock type phase type, then the followingcan be performed.

If pt_a is a double-gated type phase type, indicated by an order numberin the range 35 to 44, then the variable pt_a_const can be set to “?”.Else if pt_a is a high-gated type phase type, indicated by an ordernumber in the range 25 to 34, then the variable pt_a_const can be set to“1”. Else, pt_a is a low-gated type phase type, indicated by an ordernumber in the range 15 to 24, and pt_a_const can be set to “0”.Similarly, if pt_b is a double-gated type phase type, then pt_b_constcan be set to “?”. Else if pt_b is a high-gated type phase type, thenthe pt_b_const can be set to “1”. Else, pt_b is a low-gated type phasetype, and pt_b_const can be set to “0”.

A result of (pt_y_const=pt_op(pt_a_const, pt_b_const)) can be determinedto yield “?”, “1”, or “0”. This result can represent a gated state ofthe phase type output (assuming, as noted above, that both of the phasetype inputs are of the gated clock phase type).

If pt_y_const is equal to “?” or “0”, then the variable pt_y_temp can bereassigned to the result of the AND phase type operator, such as pt_and,of (pt_y_temp, “?”). The foregoing result can be a low-gated clock type.If a result of pt_y_const is equal to “?” or “1”, then the variablept_y_temp can be reassigned to the result of the OR phase operator, suchas pt_or of (pt_y_temp, “?”). In one implementation, the pt_or (a,b) isthe result of pt_not(pt and(pt_not(a), pt_not(b))). In other words,using De Morgan's law, an OR operator can be expressed in terms of usingAND and NOT (inverter) operations. It is noted that if pt_y_const isequal to “?”, both this step and the previous step (of reassigning thevariable pt_y_temp) are both performed. In any of the steps above, theresult of the phase type operator (e.g., pt_y) can then be set to theresult (e.g., pt_y_temp).

If the clock-gating algebra is implemented, one of the phase type inputsto a binary phase type operator can be ungated. If some binary phasetype operator with two inputs is used, and one phase type input is agated clock type phase type, and the result of an initial operation ofthe binary phase operator using both inputs is a “*”, then the gatedclock type phase type can be replaced with a corresponding ungated clocktype phase type, which can be obtained by subtracting 10, 20, or 30 fromthe order number of the gated clock type phase type, as shown in columnone of Table 6. The result of the binary phase type operator is thengenerated using the corresponding ungated clock type phase type. Forexample, if the foregoing condition is met, then a gated clock typephase type input of “C!1%*” is converted to “C!1”.

Table 9 (shown in FIG. 17M) identifies five phase type group operatorsby example function names in column one: ptg_not, ptg_and, ptg_xor,ptg_latch, and ptg_unit_delay. These phase type group operatorscorrespond to the inverter, AND gate, XOR gate, transparent latch, andunit delay circuit components. Column two of Table 9 indicates anexample syntax that may be used for the phase type group operators. Thephase type group operator syntax depicted in column two of Table 9generally follows the other syntaxes described herein. For a singleinput phase type group operator, the syntax is ptg_y=ptg_operator(ptg_input). For a dual input phase type group operator, the syntax isptg_y=ptg_operator (ptg_input1, ptg_input2). Column three of Table 9 isa table that specifies the resulting phase type group output given a setof phase type group inputs. These tables are similar to those specifiedin Table 3 for the NTF operators. The phase type group operatorsidentified in Table 9 indicate possible output referred to herein asmeta-phase type groups (meta-PTGs). A meta-phase type group is agrouping of phase type groups. The phase type group operators in Table 9provide for the possibility of three (3) meta-PTGs. These are denotedherein as GXC, GXL, and GXT. Table 10 (shown in FIG. 17N) identifies thephase type group members of each meta-PTG. Meta phase type groups areimplemented to specify results of phase type group operations thatconform to the rules specified herein. Phase type groups allow for thecompact representations of multiple waveforms because the groupidentifiers can be used to disambiguate a sequence of nondeterministicsignal transition representations that map to different phase types.

FIG. 5 is a conceptual diagram that depicts an example hierarchy ofrelationships among data constructs. Depending on the programminglanguage and particular literature, a data construct can be referred toas a class, an object, a structure, etc. This example data construct 500includes several elements or members that define the structure of theclass and behavior of the class. The structure of this data construct500 is defined by the following members: NTFs 508, CWSSs 516, phase typegroups 520, meta phase type groups 524, phase types 532, phase tags 536,mode expressions 540, mode independent phase expressions (MIPEs) 544,phase expressions 548, reduced orthogonal list of conditional MIPE pairs(ROLCMPs) 552, and phase ids 556. The behavior of the data construct 500is defined by functions or operators that operate on the depictedmembers: NTF operators 560, CWSS operators 564, phase type groupoperators 568, phase type operators 572, phase tag operators 576, modeoperators 580, MIPE operators 584, phase expression operators 588, andphase id operators 592. Subsequent figures provide additional detailregarding each illustrated member and operator.

FIG. 5 depicts a waveform set function (WSF) 504 as supporting the NTFdata construct 508. The WSF 504 is depicted with a dashed line becausethe WSF 504 may not be explicitly defined in a data construct. An NTFdata construct can be defined in a class, for example, based onassumptions that rely upon a WSF without explicitly indicating themappings from each unit of time to an NTF. But the NTF data construct508 would express the definitions indicated in Table 2.

A CWSS 516 is a sequence of NTFs 508. Together with a virtual clock 512,a CWSS 516 defines sets of waveforms 528. The virtual clock 512 is alsodepicted with a dashed line because this may not be explicitly definedin a data construct. The information for a virtual clock (e.g., timingparameters) can be assumed or implied by the CWSS data construct 516.The NTF operators 560 manipulate each NTF 508 that comprises an instanceof a CWSS 516, thereby manipulating the CWSS 516 instance.

A user applies phase tags 536 or phase expressions 548 to the primaryinputs and the outputs of clock generators in a circuit design.Operations are performed on these phase tags 536 or phase expressions548. When the operations are performed, the phase tags 536 or phaseexpressions 548 are propagated throughout a design, and the resultingphase tags 536 or phase expressions 548 can be analyzed to identifypossible design defects or particular design characteristics. A phasetag 536 or phase expression 548 is propagated throughout the circuitdesign by transforming input phase tags or input phase expressionsreceived at primary inputs and outputs of clock generators in a circuitdesign through the previously discussed look up tables so that eachoutput net of the circuit design includes a phase tag 536 or phaseexpression 548.

A phase type 532 is a generalized version of a phase tag 536. While aphase tag 536 can be associated with a particular virtual clock 512, aphase type 532 is a generalized expression representing a set ofwaveforms 528. As with the other variable types, a phase type 532 can bemanipulated through phase type operators 572. A phase type 532 isassociated with a clocked waveform set specification (CWSS) 516 and aphase type group 520.

As previously mentioned, multiple phase types 532 can be associated withthe same CWSS 516. A phase type group 520 distinguishes such phase types532, and can distinguish characteristics of signals represented by phasetypes 532, such as clock signals as compared to data signals. Certainphase type groups 520 can be constituent elements of a meta phase typegroup 524. Phase type groups 520 and meta phase type groups 524 can bemanipulated through phase type group operators 568.

Phase tags 536 and phase expressions 548 themselves are comprised oflower level data constructs (e.g., CWSSs) and also can be converted intodifferent data constructs on which operations are executed. A phaseexpression 548 is comprised of zero or more mode expressions 540 and oneor more MIPEs 544.

A mode expression 540 represents a condition in which a design canoperate among multiple modes. A mode is a Boolean function of the valueof a signal in a circuit, referred to as a mode signal. A mode signalcan be used to select between a first signal and a second signal that isdifferent from the first signal. For example, a design might include adual input multiplexer. A first input to the multiplexer might be afirst clock signal and a second input to the multiplexer might be asecond clock signal that is asynchronous to the first clock signal. Themultiplexer can receive a selector signal that causes it to selectbetween the first signal and the second signal. In this example, thedesign includes more than one mode, which can be represented via a modeexpression 540. Operations can be performed on the mode expressions 540through the mode operators 580.

A MIPE 544 is comprised of one or more phase tags 536. A MIPE 544represents a set of waveforms 528 that is a function of the sets ofwaveforms 528 represented by the constituent phase tags 536 of the MIPE544. Operations can be performed on a MIPE 544 through the MIPEoperators 584.

A phase expression 548 can be converted into a reduced orthogonal listof conditional MIPE pairs 552, designated as a ROLCMP 552. A ROLCMP 552is a data construct that enables phase expressions 556 to be convertedinto phase ids 556. A phase id 556 is a numerical handle associated withphase expressions 548, enabling phase expressions 548 to be more easilymanipulated, such as described in the U.S. Provisional PatentApplication Ser. No. 61/912,345.

A phase tag 536 represents a set of waveforms 528 via CWSSs. In somecases, a phase tag 536 can be associated with a virtual clock 512.Syntactically, if a phase tag 536 is associated with a virtual clock512, the phase tag 536 will follow a syntax which includes the name ofthe virtual clock 512. One such syntax can be represented as “ClockName@Type of Clock Signal.” For example, the phase tag 536 “A@L”designates the waveform set 528 associated with a latch clocked by theleading phase of virtual clock “A.” However, in other cases, a phase tag536 may not be associated with a virtual clock 512. For instance, thephase tag “*” designates the set of all possible waveforms 528. Phasetags 536 can be manipulated via phase tag operators 576. Phase tagoperators 576 implement operations on phase tags 536. A phase tag 536can be employed to distinguish among a type of signal, such as whether asignal is a clock signal, a data signal (e.g., latch driven signal), ora constant; a type of clock, such as a level, pulse, or delayed clockand inverted versions of each; and a phase of data, such as leading,trailing, or a combination.

As mentioned earlier, a phase type 532 is a generalized expressionrepresenting a set of waveforms 528. For example, a phase tag 536 suchas “A@L” can be generalized to the phase type “C@L,” which represents aset of waveforms 528 associated with a leading-phase-clocked latchclocked by any clock C. In some instances, a phase tag 536 conflateswith the concept of a phase type 532.

As discussed above, more than one phase type 532 can be represented byidentical CWSSs 516. Phase type groups 520 can distinguish phase types532 that are represented by identical CWSSs 516. Phase type groups 520can also be implemented to distinguish among classes of signals, such asclock signals, data signals, and combinations of clock and data signals.Phase expressions 548 can be comprised of mode expressions 540 and MIPES544. A mode expression 540 is a Boolean function with a mode as itsargument.

FIG. 6 is a flowchart of example operations for initializing an RTLcircuit design representation of phase algebra based evaluation andpropagation of compact multi-waveform representations throughout thedesign representation. At block 604, a representation of multiplewaveforms is received at each primary input and at the output of eachclock generator of an RTL circuit design representation. For instance, aphase tag or phase expression is associated with a primary input of anRTL circuit design representation. At block 608, the RTL circuit designrepresentation is initialized to prepare the RTL circuit designrepresentation to accept propagated multi-waveform representations. Theinitialization marks nets for propagation operations. At block 612, themulti-waveform representations are determined for each of the nets inthe RTL circuit design resulting from the received multi-waveformrepresentation. For example, operators are applied to determine outputphase tags based on the various circuit components modeled in the RTLcircuit design representation. At block 616, the determinedmulti-waveform representations are supplied for evaluation of the RTLcircuit design.

FIG. 7 illustrates terminology associated with the example propagationalgorithm and pseudocode to be described. A flip-flop 708 and path delayblock (PDB) 716 are referred to as boxes. The connectors 704, 712represent nets. The boxes 708, 716 can also be referred to as nodes. Theconnector 704 is the input net to a flip-flop 708, and the connector 712(“netA”) is both the output net from the flip-flop 708 and the input netto the PDB 716. The propagation algorithm determines an output phase id,which will appear at netA 712. PDB 716 can be referred to as the sinkbox of netA 712. In one embodiment, a source set and an update set canbe established. The source set and update set can be data structuresthat store information about the status of each box in a circuit designrepresentation. For example, the source set might include boxes that areto be processed by the current iteration of the propagation algorithm.The update set can include boxes that are to be processed by the nextiteration of the propagation algorithm. The propagation algorithm isfurther described in the U.S. Provisional Patent Application Ser. No.61/912,345. The propagation algorithm is also described in the U.S.patent application Ser. No. 14/327,658.

FIG. 8 is a flowchart of example operations for initializing a circuitdesign representation for phase algebra based evaluation. At block 804,a loop of operations begins for each net in an RTL circuit designrepresentation. The operations in the loop are represented by blocks808, 812, 816, 820, and 824. Block 828 is check for a terminationcondition for the loop. At block 808, it is determined whether amulti-waveform representation is already assigned to the net. If amulti-waveform representation is not already assigned to the net, theflow proceeds to block 812. Otherwise, the flow proceeds to block 824.At block 812, it is determined whether the net represents a circuitinput or a clock generator output. If the net represents a circuit inputor clock generator output, then the flow proceeds to block 816.Otherwise, the flow proceeds to block 820. At block 816, a nullindication is assigned to the net, and the flow proceeds to block 824.At block 824, each sink node associated with the net is added to a setof sink nodes to be processed. At block 820, an unknown indication isassigned to the net. The flow proceeds to block 828. At block 828, it isdetermined whether there exist any additional nets in the circuit designrepresentation. If additional nets exist in the circuit designrepresentation, then the flow returns to block 804. Otherwise, the flowproceeds to block 832. At block 832, an indication that initializationis complete is generated.

As discussed earlier, higher level data constructs (e.g., phase tag) aredecomposed into lower level data constructs (e.g., NTFs) in order toapply operations of circuit components modeled in the circuit designrepresentation. These operations often yield a sequence of NTFs or aCWSS that is converted back into a phase type in order for propagationto continue or determine an output to associate with a net for laterdefect analysis. FIG. 9 is a flowchart of example operations fordetermining an output multi-waveform representation based on an inputsequence of nondeterministic transition representations. At block 904,an input sequence of nondeterministic transition representations and aphase type group identifier for the input sequence is received. At block908, the first entry in an ordered phase type structure is selected thatassociates phase types with phase type groups. At block 912, it isdetermined whether the phase type group of the entry matches thereceived phase type group identifier. If the foregoing is false, thenthe flow proceeds to block 920. Otherwise, the flow proceeds to block916. At block 920, the next entry in the ordered phase type structure isselected, and the flow returns to block 912. At block 916, a sequence ofnondeterministic transition representations associated with the phasetype of the entry is determined. The flow proceeds to block 924 fromblock 916. At block 924, it is determined whether the input sequence isa subset of the phase type sequence. If the foregoing is false, then theflow returns to block 920. Otherwise, the flow proceeds to block 928. Atblock 928, the phase type indicated in the entry is returned.Embodiments may utilize additional logical constructs for analysis basedon various groupings. For instance, an additional logical construct canbe employed to group together certain phase type groups. The pseudocodebelow employs such a construct and refers to it as a meta phase typegroup.

FIGS. 10 and 11 are flowcharts of example operations for applyingcircuit component based operations to sequences of signal transitionrepresentations. For instance, the evaluation tool applies an operationfor an AND component, an operation for an XOR component, etc. Examplesof the component based operators include the CWSS operators cwss_and,cwss_xor, cwss_trans_latch, and cwss_trans_latch_const. As theevaluation tool analyzes each circuit component representation of thecircuit design representation, the evaluation tool invokes program codecorresponding to the circuit component representation. When the programcode is invoked, the operations begin at block 1008.

At block 1008, an operation that corresponds to the circuit componentrepresentation is applied to a first signal transition representation ofeach sequence of signal transition representations of each input net ofthe circuit component representation. The result of applying theoperation that corresponds to the component is assigned to a first slotof an output sequence. For example, if a circuit component is an ANDgate, the NTF operator ntf_and can be invoked. The ntf_and operatorreceives a first NTF and a second NTF associated with the first slot ofeach CWSS that is an input to input nets of the AND gate. The result ofapplying the foregoing can be assigned to the first slot of an outputsequence. For instance, the result of applying the ntf_and operator tothe first NTF associated with each input CWSS can be applied to thefirst slot of the output CWSS sequence. The flow proceeds to block 1012.

At block 1012, a process that iterates through each subsequent signaltransition representation of each sequence of each input net begins.

At block 1016, the operation is applied to the signal transitionrepresentations to yield a result. For instance and continuing with theAND gate example, the process iterates through the second through eighthslot of each input CWSS applied to the AND gate, invoking the ntf_andoperator. The flow proceeds to block 1020.

At block 1020, the results of the block 1016 are validated againstadjacency restrictions. For example, each output NTF can be validated toconfirm that such output NTF conforms to the NTF adjacency restrictions.If it does not conform, then the NTF 15 adjusted.

At block 1024, the validated result is assigned to the next slot of theoutput sequence. For example, a validated output NTF can be assigned tothe appropriate output CWSS slot.

At block 1028, it is determined whether there exist additionalsubsequent signal transition representations to process. If anyadditional subsequent signal transition representations remain, then theprocess returns to block 1012.

Otherwise, the flow proceeds to block 1101 in FIG. 11. At block 1101,the first element and last element of the output sequence are validatedagainst adjacency restrictions. The flow proceeds to block 1103.

At block 1103, the results of the validation are assigned to avalidation variable and an index X is set equal to 0. The index X isused to progress through each element of the output sequence.

At block 1105, it is determined whether the validation variable equalsthe element of the output sequence that corresponds to the location ofthe index X. If the validation variable equals the element of the outputsequence that corresponds to the location of the index X, then the flowproceeds to block 1119, where the output sequence is indicated.Otherwise, the flow proceeds to block 1107.

At block 1107, the validation variable is assigned to the element of theoutput sequence that corresponds to the location of the index X. Theflow proceeds to block 1109.

At block 1109, the index X is incremented. The flow proceeds to block1111.

At block 1111, it is determined whether the index X has reached the endof the output sequence by testing whether X equals the number of slots.If the foregoing is true, then the flow proceeds to block 1119 at whichthe output sequence is indicated. Otherwise, the flow proceeds to block1113.

At block 1113, the validation variable and the slot of the outputsequence that corresponds to the location of the index X are validatedagainst the adjacency restrictions. The flow proceeds to block 1115.

At block 1115, the validation result is assigned to the validationvariable. The flow returns to block 1105 from block 1115.

FIG. 12 is a flowchart of example operations for implementing thecwss_fix_latch operator. At block 1204, a previous variable is set tothe last slot of the first input sequence of the signal transitionrepresentations.

At block 1208, a control block iterates through each slot of thesequence.

At block 1212, it is determined whether the current slot is neither themiddle slot nor the last slot. If the current slot is neither the middleslot nor the last slot, the flow proceeds to block 1216. Otherwise, theflow proceeds to block 1224.

At block 1216, it is determined whether the current slot of the firstsequence indicates a representation of a completely nondeterministicsignal. If the foregoing is true, then the flow proceeds to block 1220.Otherwise, the flow proceeds to block 1224.

At block 1220, it is determined whether the next slot of the first inputsequence indicates a representation of a stable signal. If the foregoingis true, then the flow proceeds to block 1228. Otherwise, the flowproceeds to block 1224.

At block 1228, it is determined whether the previous variable indicatesa representation of a glitch free signal. If the foregoing is true, thenthe flow proceeds to block 1224. Otherwise, the flow proceeds to block1232.

At block 1224, the current slot of the output sequence is set toindicate the current slot of the first input sequence.

If it was determined that the previous variable does not indicate aglitch free signal, then the current slot of the output sequence is setto indicate a representation of a glitch free signal at block 1232. Fromeither block 1224 or block 1232, the flow proceeds to block 1225.

At block 1225, the previous variable is set to indicate the current slotof the first input sequence and the current slot is updated to the nextslot. Control flows from block 1225 to block 1236.

At block 1236, it is determined whether additional slots exist. Ifadditional slots do not exist, then the flow proceeds to block 1240, andthe output sequence is indicated. Otherwise, the flow returns to block1208.

FIG. 13 is a flowchart of example operations for implementation of thecwss_path_delay operator. At block 1304, a delay variable is set toindicate a stable signal representation. The flow proceeds to block1308.

At block 1308, control block begins to iterate through each slot in asequence of signal transition representations.

At block 1310, it is determined whether a multi-waveform expressionindicates a leading phase clock. If the foregoing is false, then theflow proceeds to block 1312. Otherwise, control flows to block 1316.

At block 1312, a variable J is set to a result of (I+((total number ofslots)/2)) modulo the total number of slots. I is the iterator variable,which references a slot of the sequence.

If it was determined at block 1310 that the multi-waveform expressiondoes not indicate a leading phase clock, then the variable J is set toequal the iterator I at block 1316.

At block 1320, it is determined whether I equals zero. If I equals zero,then the flow proceeds to block 1328. At block 1328, the Jth slot of theoutput sequence is set equal to the Jth slot of the first inputsequence. If, at block 1320, it is determined that I does not equalzero, then the flow proceeds to block 1324.

At block 1324, it is determined whether the Jth slot of the inputsequence specifies a subset of the waveform transitions specified by thedelay variable. If the foregoing is true, then the flow proceeds toblock 1336. Otherwise, control flows to block 1332.

At block 1332, the delay variable is set to indicate the representationindicated at the Jth slot of the first input sequence. The flow proceedsto block 1336.

At block 1336, the Jth slot of the output sequence is set equal to thedelay variable.

At block 1340, it is determined if there are additional slots. If thereare additional slots, then the flow returns to block 1308. Otherwise,the flow proceeds to block 1344 to indicate an output sequence.

FIGS. 14A-14B depict a flowchart of example operations forimplementation of the cwss_is_subset operator. At block 1401, it isdetermined whether the last slots in both input sequences of signaltransition representations indicate a glitch free signal. If theforgoing is false, then the flow proceeds to block 1405. Otherwise,control flows to block 1403.

At block 1405, the glitch free range variable is set to a valuedesignated OUT, which indicates that the iterator is outside a range ofslots in which the first input sequence of signal transitionrepresentations and the second sequence of signal transitionrepresentations both indicate a glitch free range variable incorresponding slots.

At block 1401, if it is determined that the last slots in both inputsequences of signal transition representations indicate a glitch freesignal, then the flow proceeds to block 1403.

At block 1403, the glitch free range variable is set to a valuedesignated MATCH, which indicates that first sequence of signaltransition representations and the second sequence of signal transitionrepresentations both indicate a glitch free signal in correspondingslots. The flow proceeds to block 1407.

Block 1407 is control block that begins a loop of operations thatiterates through each slot of the first and the second sequences ofsignal transition representations.

At block 1409, it is determined whether the waveforms represented bysignal transition representations of the current slot of the first inputsequence are a subset of the waveforms represented by signal transitionrepresentations of the current slot of a second input sequence. If theresult of the foregoing is false, then the flow proceeds to block 1413at which a value of false is returned. Conversely, if block 1409evaluates to true, then the flow proceeds to block 1415.

At block 1415, it is determined whether the glitch free range variableis set to a value of OUT. If the forgoing is true, then the flowproceeds to block 1417. Otherwise, the flow proceeds to block 1436.

At block 1417, it is determined whether the current slot in both inputsequences indicates a glitch free signal. If the foregoing is true, thenthe flow proceeds to block 1419 at which the glitch free range variableis set to a value of MATCH, and the flow proceeds to block 1421, whereit is determined whether additional slots remain to be processed.Otherwise, the flow proceeds to block 1421.

At block 1436, it is determined whether the current slot in the secondinput sequence does not indicate a glitch free signal. If the foregoingis true, then the flow proceeds to block 1438. Otherwise, the flowproceeds to block 1440.

At block 1438, the glitch free range variable is set to a value of OUT,and the flow proceeds to block 1421. At block 1421, it is determinedwhether additional slots remain. If there are additional slots, then theprocess returns to block 1407. Otherwise, the flow proceeds to block1423, at which a value of true is returned.

At block 1440, it is determined whether the current slot in the firstinput sequence does not indicate a glitch free signal. If the foregoingis true, then the flow proceeds to block 1444. Otherwise, the flowproceeds to block 1442. At block 1444, the glitch free range variable isset to a value of SUB. The value SUB designates that, within a range ofslots in which the first input sequence of signal transitionrepresentations and the second input sequence of signal transitionrepresentations both initially contained a glitch free signal, thereexists a subrange in which only the second input sequence of signaltransition representations contains a glitch free signal. If thestatement at block 1440 evaluates to false, then the flow proceeds toblock 1442.

At block 1442, it is determined whether the glitch free range variableindicates a value of SUB. If the foregoing is true, then a value offalse is returned at block 1446. Otherwise, the flow proceeds to block1421.

FIG. 15 is a conceptual depiction of circuit design evaluation thatshows generalized phase tag propagation. The design includes primaryinputs 1502, 1504, 1506, and 1507 to the design. The primary inputs 1504and 1506 can be generated by external clocks referred to as CLKA andCLKB, respectively.

The design includes three flip-flops 1512, 1514, and 1516. Eachflip-flop 1512, 1514, and 1516 can be comprised of two transparentlatches and an inverter. The design includes two path delay blocks 1522and 1524 to model signal propagation delays. The design also includesAND gates 1532 and 1534.

The evaluation tool can convert the schematic into a netlist. The nodesof the netlist correspond to the circuit components of FIG. 15. In oneembodiment, a user can provide the phase tags “A@L”, “A”, “B”, and“B@TPGF” as inputs to a general purpose computer executing a softwareembodiment of the disclosure.

A propagation process can begin application of phase tag operators, asfurther described in the U.S. Provisional Patent Application Ser. No.61/912,345. Since the circuit component associated with box 1512 is aflip-flop, in one embodiment, the flip-flop phase tag operator isinvoked. The flip-flop phase tag operator takes two phase tag inputs,“A@L” and “A.” The phase tag operator associated with a flip-flopinvokes the phase type operator associated with a flip-flop. The resultof the phase type operator associated with a flip-flop is a phase type“C@LPGF”. The phase tag operator converts a phase type result to acorresponding phase tag “A@LPGF” at the output net associated with theflip-flop. Similarly, the propagation algorithm updates the value at thePDB1 1522 output net. The propagation algorithm generates the value atthe output net of the PDB1 1522 with the operators associated with thePDB circuit component. The PDB2 1524 is similarly processed. It is notedthat the PDB1 and PDB2 elements are implemented using glitch awarealgebra. Furthermore, a phase tag operator associated with an AND gateinvokes the phase type operator associated with an AND gate, updating avalue at the AND 1534 output net to “B %*”.

As will be appreciated by one skilled in the art, aspects of the presentdisclosure may be embodied as a system, method or computer programproduct. Accordingly, aspects of the present inventive subject mattermay take the form of an entirely hardware embodiment, a softwareembodiment (including firmware, resident software, micro-code, etc.) oran embodiment combining software and hardware aspects that may allgenerally be referred to herein as a “circuit,” “module” or “system.”Furthermore, aspects of the present inventive subject matter may takethe form of a computer program product embodied in one or more computerreadable medium(s) having computer readable program code embodiedthereon.

The present invention may be a system, a method, and/or a computerprogram product. The computer program product may include a computerreadable storage medium (or media) having computer readable programinstructions thereon for causing a processor to carry out aspects of thepresent invention.

The computer readable storage medium can be a tangible device that canretain and store instructions for use by an instruction executiondevice. The computer readable storage medium may be, for example, but isnot limited to, an electronic storage device, a magnetic storage device,an optical storage device, an electromagnetic storage device, asemiconductor storage device, or any suitable combination of theforegoing. A non-exhaustive list of more specific examples of thecomputer readable storage medium includes the following: a portablecomputer diskette, a hard disk, a random access memory (RAM), aread-only memory (ROM), an erasable programmable read-only memory (EPROMor Flash memory), a static random access memory (SRAM), a portablecompact disc read-only memory (CD-ROM), a digital versatile disk (DVD),a memory stick, a floppy disk, a mechanically encoded device such aspunch-cards or raised structures in a groove having instructionsrecorded thereon, and any suitable combination of the foregoing. Acomputer readable storage medium, as used herein, is not to be construedas being transitory signals per se, such as radio waves or other freelypropagating electromagnetic waves, electromagnetic waves propagatingthrough a waveguide or other transmission media (e.g., light pulsespassing through a fiber-optic cable), or electrical signals transmittedthrough a wire.

Computer readable program instructions described herein can bedownloaded to respective computing/processing devices from a computerreadable storage medium or to an external computer or external storagedevice via a network, for example, the Internet, a local area network, awide area network and/or a wireless network. The network may comprisecopper transmission cables, optical transmission fibers, wirelesstransmission, routers, firewalls, switches, gateway computers and/oredge servers. A network adapter card or network interface in eachcomputing/processing device receives computer readable programinstructions from the network and forwards the computer readable programinstructions for storage in a computer readable storage medium withinthe respective computing/processing device.

Computer readable program instructions for carrying out operations ofthe present invention may be assembler instructions,instruction-set-architecture (ISA) instructions, machine instructions,machine dependent instructions, microcode, firmware instructions,state-setting data, or either source code or object code written in anycombination of one or more programming languages, including an objectoriented programming language such as Java, Smalltalk, C++ or the like,and conventional procedural programming languages, such as the “C”programming language or similar programming languages. The computerreadable program instructions may execute entirely on the user'scomputer, partly on the user's computer, as a stand-alone softwarepackage, partly on the user's computer and partly on a remote computeror entirely on the remote computer or server. In the latter scenario,the remote computer may be connected to the user's computer through anytype of network, including a local area network (LAN) or a wide areanetwork (WAN), or the connection may be made to an external computer(for example, through the Internet using an Internet Service Provider).In some embodiments, electronic circuitry including, for example,programmable logic circuitry, field-programmable gate arrays (FPGA), orprogrammable logic arrays (PLA) may execute the computer readableprogram instructions by utilizing state information of the computerreadable program instructions to personalize the electronic circuitry,in order to perform aspects of the present invention.

Aspects of the present invention are described herein with reference toflowchart illustrations and/or block diagrams of methods, apparatus(systems), and computer program products according to embodiments of theinvention. It will be understood that each block of the flowchartillustrations and/or block diagrams, and combinations of blocks in theflowchart illustrations and/or block diagrams, can be implemented bycomputer readable program instructions.

These computer readable program instructions may be provided to aprocessor of a general purpose computer, special purpose computer, orother programmable data processing apparatus to produce a machine, suchthat the instructions, which execute via the processor of the computeror other programmable data processing apparatus, create means forimplementing the functions/acts specified in the flowchart and/or blockdiagram block or blocks. These computer readable program instructionsmay also be stored in a computer readable storage medium that can directa computer, a programmable data processing apparatus, and/or otherdevices to function in a particular manner, such that the computerreadable storage medium having instructions stored therein comprises anarticle of manufacture including instructions which implement aspects ofthe function/act specified in the flowchart and/or block diagram blockor blocks.

The computer readable program instructions may also be loaded onto acomputer, other programmable data processing apparatus, or other deviceto cause a series of operations to be performed on the computer, otherprogrammable apparatus or other device to produce a computer implementedprocess, such that the instructions which execute on the computer, otherprogrammable apparatus, or other device implement the functions/actsspecified in the flowchart and/or block diagram block or blocks.

The flowchart and block diagrams in the Figures illustrate thearchitecture, functionality, and operation of possible implementationsof systems, methods, and computer program products according to variousembodiments of the present invention. In this regard, each block in theflowchart or block diagrams may represent a module, segment, or portionof instructions, which comprises one or more executable instructions forimplementing the specified logical function(s). In some alternativeimplementations, the functions noted in the block may occur out of theorder noted in the figures. For example, two blocks shown in successionmay, in fact, be executed substantially concurrently, or the blocks maysometimes be executed in the reverse order, depending upon thefunctionality involved. It will also be noted that each block of theblock diagrams and/or flowchart illustration, and combinations of blocksin the block diagrams and/or flowchart illustration, can be implementedby special purpose hardware-based systems that perform the specifiedfunctions or acts or carry out combinations of special purpose hardwareand computer instructions.

FIG. 16 depicts an example computer system compact multi-wave basedcircuit design evaluator. A computer system includes a processor unit1604 (possibly including multiple processors, multiple cores, multiplenodes, and/or implementing multi-threading, etc.). The computer systemincludes a memory unit 1608. The memory unit 1608 may be system memory(e.g., one or more of cache, SRAM, DRAM, zero capacitor RAM, TwinTransistor RAM, eDRAM, EDO RAM, DDR RAM, EEPROM, NRAM, RRAM, SONOS,PRAM, etc.) or any one or more of the above already described possiblerealizations of machine-readable media. The computer system alsoincludes a bus 1612 (e.g., PCI bus, ISA bus, PCI-Express bus,HyperTransport® bus, InfiniBand® bus, NuBus, etc.). The computer systemalso includes a compact multi-wave based circuit design evaluator(“evaluator”) 1621. The evaluation tool propagates compactrepresentations of multiple waveforms throughout nets of a registerlevel circuit design representation as previously described. The memoryunit 1608 may include one or more functionalities that facilitatestoring the look-up tables or other data structures for evaluating acircuit design representation based on representations of multiplewaveforms and decomposition of compact multi-waveform representationsinto sequence of nondeterministic signal transition representations. Anyone of these functionalities may be partially (or entirely) implementedin hardware and/or on the processor unit 1604. For example, thefunctionality may be implemented with an application specific integratedcircuit, in logic implemented in the processor unit 1604, in aco-processor on a peripheral device or card, etc. The processor unit1604 and the memory unit 1608 are coupled to the bus 1612. Althoughillustrated as being coupled to the bus 1612, the memory unit 1608 maybe coupled to the processor unit 1604.

While the embodiments are described with reference to variousimplementations and exploitations, it will be understood that theseembodiments are illustrative and that the scope of the inventive subjectmatter is not limited to them. In general, techniques for evaluating aregister level circuit design representation with compact multi-waveformrepresentations as described herein may be implemented with facilitiesconsistent with any hardware system or hardware systems. Manyvariations, modifications, additions, and improvements are possible.

Plural instances may be provided for components, operations orstructures described herein as a single instance. Finally, boundariesbetween various components, operations and data stores are somewhatarbitrary, and particular operations are illustrated in the context ofspecific illustrative configurations. Other allocations of functionalityare envisioned and may fall within the scope of the inventive subjectmatter. In general, structures and functionality presented as separatecomponents in the example configurations may be implemented as acombined structure or component. Similarly, structures and functionalitypresented as a single component may be implemented as separatecomponents. These and other variations, modifications, additions, andimprovements may fall within the scope of the inventive subject matter.

What is claimed is:
 1. A method for processing register transfer levelcode representing a circuit design, the method comprising: determining,by one or more processors based on the register transfer level code, aninput sequence of signal transitions associated with an input net of acomponent represented in the register transfer level code, wherein eachsignal transition represents a nondeterministic transition from a firstsignal state to one or more possible signal states; determining, atleast one of the processors based on the register transfer level code,that a subsequence of signal transitions of the input sequence indicatesat most one transition within the subsequence; and determining, by atleast one of the processors based on the component represented in theregister transfer level code and on the subsequence, an output sequenceof signal transitions derived from the input sequence of signaltransition.
 2. The method of claim 1, further comprising: determining,by one or more processors based on the register transfer level code,whether the output sequence conforms to restrictions that indicate whichsignal transitions can be adjacent to other signal transitions.
 3. Themethod of claim 2, further comprising: if the output sequence of signaltransitions conforms to the restrictions, associating, by one or moreprocessors based on the register transfer level code, the outputsequence of signal transitions with an output net of the component; andif the output sequence of signal transitions does not conform to therestrictions, modifying, by one or more processors based on the registertransfer level code, the output sequence of signal transitions toconform to the restrictions and associating the modified output sequenceof signal transitions with the output net.
 4. The method of claim 1,wherein determining that the subsequence of the input sequence indicatesat most one transition comprises: determining whether the subsequence ofthe input sequence indicates a glitch-free or a glitch-prone signal. 5.The method of claim 1, wherein determining the output sequence of signaltransitions comprises determining whether at least one signal transitionof the output sequence of signal transitions indicates anondeterministic output delay.
 6. The method of claim 1, whereindetermining the output sequence of signal transitions comprisesdetermining whether a source of at least one signal transitionrepresentation of the input sequence of signal transitions is anindeterminate delay block.
 7. The method of claim 1, wherein saiddetermining the output sequence of signal transitions comprises applyingan operation that represents a behavior of the indicated component. 8.The method of claim 1, further comprising: initializing nets of thecircuit design to identify those of the nets associated with glitch-freesignal transitions and those of the nets associated with glitch-pronesignal transitions.
 9. A computer program product for evaluating acircuit design, the computer program product comprising: a computerreadable storage medium having program instructions stored thereon, theprogram instructions comprising instructions to determine, by one ormore processors based on the register transfer level code, an inputsequence of signal transitions associated with an input net of acomponent represented in the register transfer level code, wherein eachsignal transition represents a nondeterministic transition from a firstsignal state to one or more possible signal states; instructions todetermine, at least one of the processors based on the register transferlevel code, that a subsequence of signal transitions of the inputsequence indicates at most one transition within the subsequence; andinstructions to determine, by at least one of the processors based onthe component represented in the register transfer level code and on thesubsequence, an output sequence of signal transitions derived from theinput sequence of signal transition.
 10. The computer program product ofclaim 9, wherein the instruction further comprise: instructions todetermine whether the output sequence of signal transitions conforms torestrictions that indicate which of the signal transitions can beadjacent to other of the signal transitions.
 11. The computer programproduct of claim 10, wherein the program instructions further compriseinstructions to, if the output sequence of signal transitions conformsto the restrictions, associate the output sequence of signal transitionswith an output net of the; and instructions to, if the output sequenceof signal transitions does not conform to the restrictions, modify theoutput sequence of signal transitions to conform to the restrictions andassociate the modified output sequence of signal transitions with theoutput net.
 12. The computer program product of claim 9, wherein theprogram instructions to determine that the subsequence of the inputsequence of signal transitions indicates at most one transition compriseprogram instructions to determine whether the subsequence of the inputsequence of signal transitions indicates a glitch-free or a glitch-pronesignal.
 13. The computer program product of claim 9, wherein the programinstructions to determine the output sequence of signal transitionscomprise program instructions to determine whether at least one signaltransition representation of the output sequence of signal transitionsindicates a nondeterministic output delay.
 14. The computer programproduct of claim 9, wherein the program instructions to determine theoutput sequence of signal transitions comprise program instructions todetermine whether a source of at least one signal transitionrepresentation of the input sequence of signal transitions is anindeterminate delay block.
 15. The computer program product of claim 9,wherein the program instructions further comprise program instructionsto initialize nets of the register transfer level circuit design toidentify those of the nets associated with glitch-free signaltransitions and those of the nets associated with glitch-prone signaltransitions.
 16. An apparatus comprising: a processor; and a computerreadable storage medium having stored thereon program instructionsexecutable by the processor to cause the apparatus to, instructions todetermine, by one or more processors based on the register transferlevel code, an input sequence of signal transitions associated with aninput net of a component represented in the register transfer levelcode, wherein each signal transition represents a nondeterministictransition from a first signal state to one or more possible signalstates; instructions to determine, at least one of the processors basedon the register transfer level code, that a subsequence of signaltransitions of the input sequence indicates at most one transitionwithin the subsequence; and instructions to determine, by at least oneof the processors based on the component represented in the registertransfer level code and on the subsequence, an output sequence of signaltransitions derived from the input sequence of signal transition. 17.The apparatus of claim 16, wherein the program instructions furthercomprise: instructions to determine whether the output sequence ofsignal transitions conforms to restrictions that indicate which of thesignal transitions can be adjacent to other of the signal transitions.18. The apparatus of claim 17, wherein the program instructions areexecutable to further cause the apparatus to, instructions to, if theoutput sequence of signal transitions conforms to the restrictions,associate the output sequence of signal transitions with an output netof the indicated component; and instructions to, if the output sequenceof signal transitions does not conform to the restrictions, modify theoutput sequence of signal transitions to conform to the restrictions andassociate the modified output sequence of signal transitions with theoutput net.
 19. The apparatus of claim 16, wherein the programinstructions executable to cause the apparatus to determine that thesubsequence of the input sequence of signal transitions indicates atmost one transition comprise program instructions executable to causethe apparatus to determine whether the subsequence of the input sequenceof signal transitions indicates a glitch-free or a glitch-prone signal.20. The apparatus of claim 16, wherein the program instructionsexecutable to cause the apparatus to determine the output sequence ofsignal transitions comprise program instructions executable to cause theapparatus to determine whether a source of at least one signaltransition representation of the input sequence of signal transitions isan indeterminate delay block.